Certain magnetic resonance balanced steady-state free precession (bSSFP) imaging applications require large and frequent changes in the gradient waveforms from one TR to the next. One such application of particular interest in our research group is bSSFP phase-contrast (PC) magnetic resonance imaging (MRI), which utilizes two or more images acquired with different gradient first moments at the echo time (TE). This can be achieved by switching between a positive and negative bipolar gradient pulse played before or after the data acquisition window, or by inverting the sign of the imaging gradients themselves. Unfortunately, different gradient wave-forms will in general produce slightly different residual eddy-current fields, which in turn give rise to small differences in the precession angle. As a result, switching between two waveforms on a TR-by-TR basis can produce an “oscillating” steady state for spins near on-resonance, resulting in severe image artifacts. For this reason, bSSFP PC-MRI has been limited to noninterleaved imaging, which limits the application of this technique to non real-time acquisitions, and makes the measurements more susceptible to patient motion.
It has been shown that eddy-current-induced bSSFP signal distortions for on-resonance spins can be mitigated by pairing the waveforms. See Bieri O., et al., “Analysis and compensation of eddy currents in balanced SSFP,” Magn. Reson. Med. 2005;54:129-137; and U.S. Pat. No. 7,046,004; the entire contents of both of which are incorporated herein by reference. Waveform pairing can ensure that any eddy-current-induced change in precession angle in one TR is simply reproduced in the following TR. This tends to bring spins that are near on-resonance back to the same starting point, regardless of the precise value of the small and unknown eddy-current-induced change in precession angle.
Waveform pairing has been applied to several bSSFP imaging applications including phase-contrast imaging, however, paired interleaved bSSFP introduces signal distortions approximately halfway between the center of the bSSFP passband and the signal nulls. In order to suppress within-band signal distortions in paired bSSFP, Markl et al. proposed a “double average” (dAVE) acquisition scheme in which every phase-encode is acquired twice and the complex signals are averaged. See, Markl M., et al., “Double average parallel steady-state free precession imaging: optimized eddy current and transient oscillation compensation,” Magn. Reson. Med., 2005;54:965-974. Such dAVE imaging has the advantage that it simultaneously suppresses signal distortions due to both unbalanced eddy-currents and off-resonance-induced signal oscillations during the transient toward steady state, at the cost of doubling the acquisition time (for a given phase-encode acquisition scheme), however, twice the number of acquisitions and thus double the time is required.